1. Field of the Invention
This invention relates to a noise reduction method for reducing 1/f noise of a semiconductor device which includes an element having an impedance and a MOSFET (Metal Oxide Semiconductor Field Effect Transistor).
2. Description of the Related Art
Usually, noise components of a semiconductor device include 1/f noise and Johnson noise, and where the semiconductor device is a sensor or a like element of the temperature detection type for use with an infrared detector, thermal fluctuation noise caused by thermal energy inputted to it is produced additionally.
In order to improve the signal to noise ratio (hereinafter referred to simply as S/N ratio) of a semiconductor device, in a device which at least includes an element having an impedance and a MOSFET, which is reputed to have high 1/f noise, the 1/f noise in particular above must be reduced.
It is to be noted that (S/N ratio)=(effective value of signal)/(effective value of noise).
If noise of a semiconductor device is noise on current, then noise current spectrum density Sn1/f [A.sup.2 /Hz] and noise current effective value In1/f [A] representative of the magnitude of 1/f noise can be represented by the following expressions: EQU Sn1/f=K.multidot.Ib.sup.2 /f (1) EQU In1/f=(.intg..sub.f1.sup.f2 Snidf).sup.1/2 ={K.multidot.Ib.sup.2.multidot.1n(f2/f1)}.sup.1/2 (2)
where K is the 1/f noise factor, Ib is the bias current [A] to flow through the semiconductor device, f is the frequency [Hz], f1 is the lowest frequency [Hz] in a measurement band, and f2 is the highest frequency [Hz] in the measurement band.
As seen from expressions (1) and (2), noise current spectrum density Sn1/f [A.sup.2 /Hz] and noise current effective value In1/f [A] of 1/f noise rely upon the value of bias current Ib to flow through the semiconductor device, and if bias current Ib increases, also 1/f noise increases.
Meanwhile, noise current effective value InJ of Johnson noise and noise current effective value Inth of thermal fluctuation noise can be represented, when an infrared sensor of the bolometer type used in an infrared detector or the like as a semiconductor device is taken as an example, by the following expressions:
InJ={4.multidot.k.multidot.T.multidot.(f2-f1)/R}.sup.1/2 (3) EQU Inth=Ib.multidot..alpha..multidot.T.multidot.(k/Cth).sup.1/2 (4)
where R is the total resistance value [.OMEGA.] of the semiconductor device, k is the Boltzmann's constant [eV/K], T is the temperature [K] upon measurement, .alpha. is the temperature coefficient of the bolometer type infrared sensor resistance [1/K], and Cth is the heat capacity [J/K].
By the way, the magnitude of signal component Is outputted from the semiconductor device can be represented, where a bolometer type infrared sensor which is used with an infrared ray detector or the like is taken as an example (refer to, for example, Brugess R. Johnson et al., "Silicon Microstructure Superconducting Microbolometer Infrared Arrays", SPIE, Vol. 2,020, Infrared Technology XIX, 1993), by the following expression: EQU Is=.alpha..multidot..DELTA.T.multidot.Ib (5)
where .alpha. is the temperature coefficient [1/K] of the resistance of the bolometer type infrared sensor, .DELTA.T is the temperature rise [K] of a diaphragm when an object of measurement having a temperature difference of 1.degree. K from the surroundings is observed, and Ib is the bias current [A] to flow through the semiconductor device.
Using noise current effective value In1/f of 1/f noise, noise current effective value InJ of Johnson noise and noise current effective value Inth of thermal fluctuation noise, the S/N ratio of the semiconductor device can be represented by the following expression: ##EQU1##
Of the noise components of a semiconductor device, the thermal fluctuation noise has a value much lower than those of the Johnson noise and the 1/f noise, and therefore, the thermal fluctuation noise can be ignored. The thermal fluctuation noise is noise peculiar to a sensor of the temperature detection type such as an infrared detector, and matters only when the Johnson noise and the 1/f noise are very low.
By the way, when bias current Ib to flow through the semiconductor device is low, Johnson noise becomes a principal noise component of noise of the semiconductor device. On the other hand, when bias current Ib to flow through the semiconductor device is high, since also the 1/f noise increases as bias current Ib increases, the 1/f noise becomes a principal noise component of the semiconductor device.
In a range of bias current Ib in which Johnson noise is outputted as a principal noise component, if bias current Ib is increased, then signal component Is increases, and therefore, the S/N ratio can be improved. This is due to the fact that, as seen from expression (3) above, the Johnson noise is fixed without relying upon bias current Ib.
However, if bias current Ib is further increased to a value in another range of bias current Ib in which 1/f noise is outputted as a principal noise component, then improvement of the S/N ratio can no longer be expected. This arises from the fact that, if bias current Ib increases, then simultaneously as signal component Is increases as seen from expression (5), also the 1/f noise increases as seen from expression (2).
Accordingly, in order to improve the S/N ratio, signal component Is must be increased while suppressing an increase of the noise components.
This is described taking a bolometer type infrared sensor as an example. In order to increase signal component Is, as can be seen from expression (5), temperature coefficient .alpha., temperature rise .DELTA.T of the diaphragm and bias current Ib should be increased.
In order to increase temperature rise .DELTA.T of the diaphragm, it is necessary to reduce the thermal conductance or increase the absorption factor of infrared rays of the diaphragm to improve the sensitivity of the sensor. Meanwhile, in order to increase temperature coefficient .alpha. of the resistance, it is necessary to develop an infrared sensor of the bolometer type having a high temperature coefficient. However, the countermeasures just described cannot be realized readily because a high development cost and a long development time are required. Therefore, in order to increase signal component Is, the most easy and effective countermeasure is to increase bias current Ib.
On the other hand, in order to suppress an increase of noise components, a method such as forming each element of a semiconductor device from a material which produces a minimized amount of noise or to devise the structure of elements to suppress an increase of noise seem to be applicable.
For example, since the magnitude of Johnson noise relies upon the resistance value of an element, production of Johnson noise can be suppressed by making the resistance value of the element high. However, if the resistance value of the element is made high, then the operating voltage of the element becomes high and the margin for the dielectric strength is reduced, and consequently, the applied voltage is limited and bias current Ib cannot be increased. Therefore, signal component Is cannot be increased.
Meanwhile, although the 1/f noise can be reduced by forming the element from a material having a low 1/f noise coefficient, this gives rise to another problem in that temperature coefficient .alpha. of the resistance and so forth are deteriorated.
Another method of devising the structure of an element to suppress the noise production amount is disclosed, for example, in Japanese Patent Laid-Open No. 186253/96. In the conventional noise reduction method disclosed in Japanese Patent Laid-Open No. 186253/96, impurity diffusion regions are formed at locations spaced from the surface of a silicon substrate and are used as the source and the drain such that the channel may not contact with a gate oxide film to reduce the 1/f noise of the MOSFET.
With such a conventional noise reduction method for a semiconductor device as described above, since the noise increases and the S/N ratio cannot be improved in the range of bias current Ib in which 1/f noise is outputted as a principal noise component. Bias current Ib must be limited to a range within which 1/f noise does not become a principal noise component.
Accordingly, in order to obtain a high S/N ratio in this condition, an increase of signal component Is is inevitable. However, in order to increase signal component Is, it is necessary to develop a material having a high temperature coefficient of the resistance or a new element or structure for improving the sensitivity. Consequently, there is a problem that a high cost and much time are required in order to develop a material or a structure or establish a production method which satisfies this requirement.
Here, while it may be a possible idea to use, as a material for the element, a metal oxide or the like which has a high temperature coefficient of the resistance such as vanadium oxide (VOx), in order to form an element from vanadium oxide, a special manufacture line is required and the production cost is increased because it is difficult to use a manufacture line which is used for formation of elements using silicon.
Also where an element is formed from a material having a low 1/f noise coefficient to reduce noise components to improve the S/N ratio or where the structure of an element is devised to reduce noise components to improve the S/N ratio, in order to develop the material and establish a manufacture method, a high cost and much time are required. Consequently, there is a problem that the production cost becomes high. If a special structure which reduces the 1/f noise coefficient as in a MOSFET which is disclosed, for example, in Japanese Patent Laid-Open No. 186253/96 is adopted, then the structure and the production process of the element are complicated. Consequently, an increase of the production cost cannot be avoided.